Antiplugging control for motors



Jan. 17, 1950 J. ElSENBERG 2,494,611

, NOW BY JUDICIAL CHANGE OF NAME J. G. ELWOOD ANTIPLUGGING CONTROL FOR MOTORS Filed March 29, 1946 2. Sheets-5heet l ATTORNEY EISENBERG NOW BY JUDICIAL CHANGE OF NAME J. G. ELWOOD ANTIPLUGGING CONTROL FOR MOTORS Jan. 17, 1950 2 Sheets-Sheet 2 Filed March 29, 1946 TIME DELAY RELAY //v1 /v fog, J06 I05 6. 5/554/56'f6 A/0W 57.100/6/06 6609/1 66 OF/V/VME ATTORNEY Patented Jan. 17, 1950 ANTIPLUGGING CONTROL FOR MOTORS Julius G. Eisenberg, Chicago, 111., now by judicial change of name Julius G. Elwood, assignor to Carnegie-Illinois Steel Corporation, a corporation of New Jersey Application March 29, 1946, Serial No. 658,054

2 can". (Cl. 318-262) This invention relates to an anti-plugging control for a motor and more particularly to such a control for hoisting motors used in steel mills.

Previous to my invention, constant trouble was experienced with crane hoist motors because the existing means of protection against plugging are ineflective under certain conditions. The term plugging of a motor" means reversing the direction of rotation 02 the motor before the motor has come to a complete stop from its original direction of rotation. This causes a greatly intensified current to flow through the armature coils of the motor which, it not controlled, may damage the motor. The high currents are due to the fact that the C. E. M. F. (counterelectromotive force), which is the stabilizing element of the motor speed, is in the same direction as the applied E. M. F. at the instant of plugging and therefore there is no electrical restraining force. The C. E. M. F. can be of large proportions when there is a large overhauling load on the motor. This C. E. M. F. plus the unrestrained line voltage sets up high stresses within the coils of the motor armature which often wreck the armature and also the field windings.

In case the motor speed becomes excessive while lowering a heavy load, the operator usually becomes excited and his first thought is to throw the motor into the reverse direction in order to stop the load. Most cranes are built without any protection against plugging the hoist motor although in some instances partial protection under certain conditions is provided for.

It is therefore an object of my invention to provide a control for preventing the harmful effects of plugging hoist motors.

Another object is to provide protection of hoist motor armatures against overloading stresses due to acceleration of the load being lowered.

These and other objects will be more apparent after referring to the following specification and attached drawings, in which:

Figure 1 is a schematic wiring diagram of my invention;

Figure 2 is a fragmentary schematic wiring diagram of a second embodiment; and

Figure 3 is a fragmentary: schematic wiring diagram of a third embodiment of my invention.

Referring more particularly to Figure l of the drawings, the power circuit is shown in heavy lines and the control circuit is shown in light lines. Power is supplied to the motor armature 2 switch 3. Power is supplied to the control circuit through combined knife switch and fuse II. The control circuit is provided with a master switch l2 having a number of contacts operated by cams whose positions are controlled by the master switch handle. As shown, the master switch is provided with-three lowering positions and 'four hoisting positions. The switch is provided with contacts l6, I8, 20, 22, 24, 2c and 28. When the operator places the master switch handle in the first position on the lowering side, the contacts l6, I3, 22, 26 and 28 close, this being indicated by the X adjacent the contacts under position I. This energizes the contactor coils 30, 32, 34, 38, 42 and 44 to close contacts 30, 32, 34, 34, 38, 42 and44 and to open contact 38. In this position the motor armature 2 is connected to the power source 4, 6 in series with the dynamic braking resistance 46 which limits the armature current. The motor series field 48 is connected to the power line in series with the magnet coil 50 of the motor brake and resistor 52. Thus the motor is connected and operated as a shunt motor which gives the series 2 from the power source 4, 6 through the knife motor the ability to hold back the load in lowering by becoming a generator. The series brake releases when current flows through its coil 50.

In the second lowering position the contacts closed in the first lowering position remain closed with the exception of contact 28 which opens, thus deenergizing coil 44 to open contact 44. This inserts the resistance 54 in series with the series fleld to reduce the field current, thus increasing the lowering speed of the motor.

In the third position, contact 2 is open which deenergizes coil 42 to open contact 42. This inserts resistor 56 in series with the series fleld to reduce the field current still further, thus increasing the lowering speed of the motor.

For the first position of the master switch on the hoist side, the switch contacts i6, 20 and 22 close, thus energizing coils 30, 36 and 38 to close contacts 30, 36 and 38 and to open contact 38. This places the motor armature 2, series field 43, magnet coil 50 and resistances 52, i4 and 56 in series. The hoisting torque of the motor is limited by the resistances 52, 54 and 56.

When the master switch is placed in the second hoisting position, contact 24 is closed in addition to the contacts previously closed. This energizes coil 40 which in turn closes contact 40 to short out resistance 52 to increase the hoisting torque of the motor and accelerate it to a higher speed.

In the third position, the additional contact 26 is closed, thus energizing coil 42 which closes contact 92 to short out the resistance 68. This further increases the hoisting speed.

In the fourth position, contact 28 is closed in addition to the contacts previously closed. This energizes coil 44 which closes contact 44 to short out resistance 64 and still further increase the hoisting speed.

The operation so far described is standard, it being understood that overload relays and limit switches may be applied to the power circuit, while under-voltage relays may be applied in the control circuit.

My invention includes a coil 58 of an anti-plugging relay which is connected across the motor armature 2 in series with a rectifier 68 which permits flow of the current only in the direction of the counter electromotive force during lowering. The relay 58 is provided with two normally closed contacts 58 and 68, the first being in the line energized by the closing of contact 20 and the second in the line energized by the closing of contact 22. As pointed out above, the motor is connected as a shunt motor during lowering of the load and the potential energy oi the load is converted into electrical energy which is dissipated in the resistances in the armature and field circuits. A small amount of this energy will be bypassed by the rectifier 60 to energize the coil 68 which causes the contactors 68 and 58 to open and remain open as long as the motor is rotating in the lowering direction. It the motor is suddenly plugged, that is, placed in the hoisting position, the open contacts 68 and 58 prevent the coils 26 and 38 from being energized and attempting to make the motor raise the load while it is still lowering. Since coil 88 is deenergized, normally closed contact 88 will be closed. Thus, the kinetic energy of the load is absorbed in the dynamic braking resistance 46 as heat energy, and also in the brake lining and brake wheel as heat energy. Stopping the load in this fashion imposes the minimum strain on the motor. When the motor comes to a complete stop the coil 58 deenergizes closing the contacts 58 and 58 and permitting the motor to hoist the load in accordance with the position 01 the master switch.

Figure 2 shows a second embodiment oi my invention. Only that part of the wiring diagram relating to contacts I8, 28 and 22 of the master switch is shown, it being understood that the remainder of the wiring is like that shown in Figure 1, except that relay 58 and rectifier 68 are omitted. In this embodiment a relay coil 62 is connected in parallel with relay coils 32 and 84. Coil 62 is provided with contacts 62 and 62 which are located in the same positions as contacts 58 and 58 in Figure 1. When the main switch handle is moved from a lowering position, contact l8 opens, thus deenergiing relay 82, closing contacts 62 and 62 after a definite time, thus preventing the motor from attempting to hoist the load while it is still lowering. The kinetic energy of the load is absorbed in the dynamic braking resistance and in the brake in the same manner as in the first embodiment. The relay 62 is timed to close contacts 62 and 62 after the motor has come to a stop.

Figure 3 discloses a third embodiment of my invention, only a portion of the power circuit is disclosed, the remainder of the wiring being sim- 4 ilar to that shown in Figures 1 and 2. In this embodiment a relay coil 64 is connected in the lowering circuit to operate the contacts 62 and 62 shown in Figure 2. Relay 64 is a current operated time delay relay whose coil is energized by the current in the lowering circuit and whose contacts 62 and 62 perform the same functions as in Figure 2. When the lowering current is interrupted, relay 64 is deenergized, but its contacts 62 and 62 remain open until the motor comes to a standstill.

I claim:

1. An anti-plugging control for a hoisting motor or the like having a power circuit for supplying power to the motor and a control circuit for controlling the rotation of the motor which control comprises a dynamic braking resistance in the power circuit, an anti-plugging relay coil connected across the motor armature, a rectifier in series with said relay coil, said rectifier permitting fiow of current only in the direction of the counter electromotive force during lowering, contacts in the control circuit operable by said relay, said contacts being closed during rotation of the motor in a hoisting direction, said rectifier passing sufiicient current to energize said relay when lowering and to keep said relay energized when the lowering circuit is interrupted to thereby hold open said contacts and prevent rotation of said motor in a hoisting direction until the kinetic energy of the load is absorbed in the dynamic braking resistance.

2. An anti-plugging control for a hoisting motor or the like having a power circuit for supplying power to the motor and a control circuit ior controlling the rotation of the motor which control comprises a dynamic braking resistance in the power circuit, a magnet coil for a brake in series with said motor, an anti-plugging relay coil connected across the motor armature, a rectifier in series with said relay coil, said rectifier permitting flow of current only in the direction of the counter electromotive force during lowering, contacts in the control circuit operable by said relay, said contacts being closed during rotation of the motor in a hoisting direction, said rectifier passing sufficient current to energize said relay when lowering and to keep said relay energized when the lowering circuit is interrupted to thereby hold open said contacts and prevent rotation of said motor in a hoisting direction until the kinetic energy of the load is absorbed in the dynamic braking resistance and by the brake.

JULIUS G. EISENBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 13,019 Hall et al Sept. 7, 1909 1,438,649 James Dec. 12, 1922 1,515,232 Whittingham Nov. 11, 1924 1,785,490 Merrill Dec. 16, 1930 1,828,564 Hardesty Oct. 20, 1931 2,057,909 Newman Oct. 20, 1936 2,370,047 King Feb. 20, 1945 

